Optical data storage systems, such as optical disk drives, allow the storage of large quantities of data on the optical medium. The data is accessed by focusing a laser beam onto the recording layer of the medium and then detecting the reflected light beam.
In write-once read-many (WORM) systems, the laser writes data by making permanent marks on the recording layer. Once the data is recorded onto the medium it cannot be erased. The data in a WORM system is detected as a change in reflectivity between the written marks and unwritten regions between the marks. In an ablative WORM system, the laser writes data by melting portions of the recording layer, resulting in physical pits in the recording layer.
In contrast to an ablative WORM system, a phase-change WORM system uses phase-change alloys as the recording material and the laser writes data by locally converting the phase-change media from one structural phase (e.g., amorphous) to a second structural phase (e.g., crystalline). This is done when the amorphous region is heated and maintained at or above its crystallization temperature, or alternatively melted and slowly cooled until the region is crystallized. The WORM function is achieved because the medium is designed so that the second structural phase does not convert back easily to the first structural phase. Another type of phase-change WORM system uses mixing or alloying of spatially separate metallic layers. During the write process, the laser heats a local region of the medium to cause inter-diffusion of the initially separate layers, resulting in a mixed alloy. The data in both types of phase-change WORM systems is detected as a change in reflectivity between a non-written region and a written region on the medium.
To increase the storage capacity of an optical disk, multiple recording layer systems have been proposed. An optical disk having two or more recording layers may be accessed at the different spatially separated recording layers by changing the focal position of the lens. Examples of this approach include U.S. Pat. Nos. 5,202,875; 5,097,464; and 4,450,553. One problem with using multiple recording layers in phase-change WORM disks is that the conventional WORM materials are highly absorptive of light. Multiple recording layer disks require that the intermediate recording layers between the disk surface onto which the laser light is incident and the last or farthest recording layer from that surface be light transmissive. Because the conventional phase-change and alloying WORM materials tend to absorb a high percentage of the light, it is not possible to write on the farther recording layers. If the intermediate recording layers are made thinner to make them transparent, they then lack sufficient reflectivity and/or signal contrast (the difference in reflectivity between the crystalline and amorphous phases of phase-change alloys) so that they fail to function as phase-change WORM recording layers.
Japanese published patent application 59-210543 by Morinaka describes a laminated optical media structure for multiplexed optical recording. The structure is illustrated with two individual light absorbing layers, without any immediately adjacent dielectric layers. The two light absorbing layers are not separated by a sufficiently thick spacer layer because the spacer layer is deposited by vapor deposition, which has too low of a deposition rate to form the spacing thickness needed for a multiple recording layer system.
What is needed is a multiple data surface system that provides a good signal from the recording layers closer to the incident laser light, as well as allowing writing and erasing with reasonable laser power on all recording layers.